Added options to save and load model weights/parameters

[comadan]

Added options to the command line tool for saving the model weights/parameters w0, w, v. This way a trained model can be reused.

I've never done any c++ programming or used github really. I also only have a high level understanding of libFM. So there's a decent chance these changes are very wrong...

[srendle]

Thanks for your contribution. I quickly scanned your code and it might work. However, using this code is likely to cause problems for other users. Your code only works correctly if the id-range and mapping in the training and test files is not changed (i.e. no new features, feature-id mapping should remain unchanged). Moreover, when running MCMC, it only recovers the last state of the chain. This might be unexpected for common users and might generate lots of questions.

I won't merge the code into the main branch because it might cause problems for inexperienced users. Users who are aware of the issues above can try your load/save code.